Diamine compound containing triazine group, polyamic acid synthesized from the diamine compound and lc alignment film prepared from the polyamic acid

ABSTRACT

Diamine compound containing specific triazine group, polyamic acid obtained by reacting the diamine compound and tetracarboxylic dianhydride, and liquid crystal alignment film obtained by coating and imidizing the polyamic acid. The liquid crystal alignment film has good heat-resistance, high transparency in visible light region and improved voltage holding ratio. Also, pretilt angle is easily controlled over broad range.

TECHNICAL FIELD

The present invention relates to a diamine compound containing atriazine moiety, a polyamic acid prepared from the diamine compound, anda liquid crystal alignment film produced from the polyamic acid. Moreparticularly, the present invention relates to a diamine compoundcontaining a specific triazine moiety, a polyamic acid prepared byreacting a diamine component (a) including the diamine compound and anacid dianhydride (b), and a liquid crystal alignment film produced byimidizing the polyamic acid.

BACKGROUND ART

Conventional polyimide resins for liquid crystal alignment films areprepared from the polycondensation of aromatic acid dianhydrides, suchas pyromellitic acid dianhydride (PMDA) and biphthalic acid dianhydride(BPDA), and aromatic diamines, such as p-phenylenediamine (p-PDA),m-phenylenediamine (m-PDA), 4,4-methylenedianiline (MDA),2,2-bisaminophenylhexafluoropropane (HFDA),m-bisaminophenoxydiphenylsulfone (m-BAPS),p-bisaminophenoxydiphenylsulfone (p-BAPS),4,4-bisaminophenoxyphenylpropane (BAPP),4,4-bisaminophenoxyphenylhexafluoropropane (HF-BAPP) and the like.

Polyimide liquid crystal alignment films produced from the conventionalaromatic acid dianhydrides and aromatic diamines are excellent inthermal stability, chemical resistance and mechanical properties, butare poor in electrooptical properties, as well as in transparency andsolubility, due to the formation of a charge transfer complex. Inefforts to solve these problems, Japanese Patent Laid-open No. Hei11-84319 discloses a polyimide-based liquid crystal alignment film intowhich an alicyclic acid dianhydride monomer or an alicyclic diamine isintroduced, and Japanese Patent Laid-open No. Hei 06-136122 discloses apolyimide-based liquid crystal alignment film having an increasedpretilt angle of a liquid crystal and improved stability produced byusing a functional diamine having a side chain or a functional aciddianhydride having a side chain.

According to a study undertaken by the present inventors, since theconventional polyimide-based alignment films use polyimides unsuitablefor surface tension and polarity to obtain a high pretilt angle ofliquid crystals, the spreadability of liquid crystals upon injection ofthe liquid crystals is poor, causing many defects in the final liquidcrystal alignment film, and the controllable range of the pretilt angleis not increased to a satisfactory level.

As demand for liquid crystal display devices has recently increased,there is a continuing need for high quality display devices. Further, astechnologies for large-scaled liquid crystal display devices have maderemarkable progress, alignment films having a high productivity are moreand more required. Thus, there is an urgent need in the art to develop aliquid crystal alignment film which facilitates the pretilt anglecontrol of a liquid crystal over a broad range of angles and has fewdefects when applied to LCD fabrication processes and shows excellentelectrooptical properties, high reliability and superior spreadabilityof a liquid crystal.

DISCLOSURE OF THE INVENTION

The present inventors have earnestly and intensively conducted researchto solve the above-mentioned problems. As a result, the presentinventors have found that an alignment film produced from theimidization of a polyamic acid, which is prepared by using a novelspecific diamine compound containing a highly polar triazine moiety, canmake a desired pretilt angle of a liquid crystal within the range of1˜90° and shows excellent alignment properties of a liquid crystal,excellent electrooptical properties, superior printability and superiorspreadability of a liquid crystal, thus accomplishing the presentinvention.

Therefore, it is a feature of the present invention to provide a liquidcrystal alignment film having superior printability, superiorspreadability of a liquid crystal, a high pretilt angle of a liquidcrystal and few defects when applied to LCD fabrication processes, whichis produced by using a novel specific triazine-based diamine compound.

In accordance with the feature of the present invention, there isprovided a diamine compound containing a triazine moiety, represented byFormula 1 below:

wherein A is a direct bond,—O— or —COO—; B is a direct bond, —O—, —COO—,—CONH— or —OCO—; and C is a C_(1˜30) linear, branched or cyclicmonovalent organic group, or a combined form thereof.

In accordance with the feature of the present invention, there isfurther provided a polyamic acid prepared by reacting a diaminecomponent (a) and an acid dianhydride (b), the diamine componentincluding 0.1 mole % or above of the diamine compound of Formula 1 basedon 100 mole % of the diamine component.

In accordance with the feature of the present invention, there is yetfurther provided a liquid crystal alignment film produced by dissolvingthe polyamic acid in a solvent to obtain a liquid crystal aligningagent, coating the aligning agent onto a substrate, and entirely orpartly imidizing the coating.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a ¹H-NMR spectrum of a diamine compound prepared inPreparative Example 3 of the present invention; and

FIG. 2 is a differential scanning calorimetry (DSC) thermogram of adiamine compound prepared in Preparative Example 3 of the presentinvention.

BEST DESCRIPTION FOR CARRYING OUT THE INVENTION

The present invention will now be described in more detail.

Preferred triazine moiety-containing diamine compounds of Formula 1 arethose wherein the substituent C is a linear or branched aliphatichydrocarbon group, a saturated cyclic hydrocarbon group, a cyclichydrocarbon group containing at least one carbon-carbon double bond, afused saturated or unsaturated cyclic hydrocarbon group, or a groupselected from the following groups:

wherein X₁ and X₂ are each independently —H, —CH₃, —CF₃, —F, —Br, —Cl,—CN, —OH, or —NO₂.

The aliphatic hydrocarbon group, saturated cyclic hydrocarbon group,cyclic hydrocarbon group containing at least one carbon-carbon doublebond, and fused saturated or unsaturated cyclic hydrocarbon group may besubstituted with at least one group selected from the group consistingof —H, —CH₃, —CF₃, —F, —Br, —Cl, —CN, —OH and —NO₂.

As diamine compounds usable in the present invention, there areexemplified the following compounds:

The present invention also provides a polyamic acid having a repeatingunit represented by Formula 2 below:

wherein x is a tetravalent aromatic or alicyclic organic group, and z isa divalent organic group originating from the diamine compound ofFormula 1 or a divalent organic group originating from an aromatic orpolysiloxane-based diamine.

Specifically, the polyamic acid of the present invention is prepared byreacting a diamine component (a) and an acid dianhydride (b), thediamine component including 0.1 mole % or above of the diamine compoundof Formula 1 based on 100 mole % of the diamine component.

The present inventors have found that the use of a specific diaminecompound containing a triazine moiety as the diamine component for thepreparation of the polyamic acid widens the controllable range of apretilt angle of the final polyimide, facilitates the control of thepretilt angle, and allows to exhibit excellent alignment properties.

The content of the diamine compound of Formula 1 in the diaminecomponent (a) is in the range of 0.1˜100 mole %, preferably 1˜60 mole %,and more preferably 2˜30 mole %, based on 100 mole % of the diaminecomponent (a).

The pretilt angle is dependent on the content of the diamine compound ofFormula 1. Depending on the mode of liquid crystal display devices, thediamine compound may be used alone to prepare the polyamic acid, afterwhich the polyamic acid is imidized to produce a liquid crystalalignment film. Optionally, there may be further added an aromaticdiamine compound and/or a polysiloxane-based diamine compoundrepresented by Formulae 3 and 4 below, respectively:H₂N—Y—NH₂   (3)

wherein Y is a divalent aromatic organic group,

wherein R₁, R₂, R₃ and R₄ are each independently a C_(1˜10) alkyl,alkoxy or aryl group, and R₅ and R₆ are each independently a C_(1˜10)alkylene group.

In Formula 3 above, Y is preferably a divalent organic group selectedfrom the group consisting of the following groups:

Preferred examples of aromatic cyclic diamine compounds of Formula 3include, but are not limited to, p-phenylenediamine (p-PDA),4,4-methylenedianiline (MDA), 4,4-oxydianiline (ODA),m-bisaminophenoxydiphenylsulfone (m-BAPS),p-bisaminophenoxydiphenylsulfone (p-BAPS),2,2-bisaminophenoxyphenylpropane (BAPP), 2,2-bisaminophenoxyphenylhexafluoropropane (HF-BAPP) and the like.

A preferred example of polysiloxane-based diamine compounds of Formula 4includes the following compound:

wherein n is an integer of 1 to 10.

The addition of the aromatic diamine compound of Formula 3 isadvantageous in that the final polyimide liquid crystal alignment filmhas an increased mechanical strength. The addition of thepolysiloxane-based diamine compound of Formula 4 is advantageous in thatthe final polyimide liquid crystal alignment film has improvedadhesiveness, leading to increased mechanical strength.

The amount of the aromatic cyclic diamine, the polysiloxane-baseddiamine or a mixture thereof used is in the range of 0˜99.9 mole %,preferably 40˜99 mole % and more preferably 70˜98 mole %, based on thetotal amount of the diamine component (a). In the case of the mixture ofthe aromatic cyclic diamine and the polysiloxane-based diamine, themixing molar ratio of the aromatic cyclic diamine to the polysiloxanediamine is between 0.5:99.5 and 99.5:0.5.

The acid dianhydride component (b) used for the preparation of thepolyamic acid according to the present invention is an aromatic cyclicacid dianhydride represented by Formula 5 below:

wherein X is a tetravalent aromatic cyclic organic group; an alicyclicacid dianhydride represented by Formula 6 below:

wherein X′ is a tetravalent alicyclic organic group; or a mixturethereof.

Preferred is a mixture of the aromatic cyclic acid dianhydride and thealicyclic acid dianhydride wherein the mixing molar ratio of thearomatic cyclic acid dianhydride to the alicyclic acid dianhydride isbetween 1:99 and 99:1.

The substituent X in Formula 5 above is preferably a tetravalentaromatic cyclic organic group selected from the following groups:

The substituent X′ in Formula 6 is preferably a tetravalent alicyclicorganic group selected from the following groups:

wherein X₁, X₂, X₃ and X₄ are each independently —H, —CH₃, —CF₃, —F,—Br, —Cl, —CN, —OH or —NO₂.

The aromatic cyclic acid dianhydride which is used to prepare thepolyamic acid of the present invention causes an alignment film (appliedto a thickness of about 0.1 μm) to be resistant to a rubbing process,which is carried out in order to drive liquid crystals in one direction,to be heat resistant to high temperature processes (200° C. or above),and to be resistant to chemicals.

Examples of aromatic cyclic acid dianhydrides include, but are notlimited to, pyromellitic acid dianhydride (PMDA), biphthalic aciddianhydride (BPDA), oxydiphthalic acid dianhydride (ODPA),benzophenonetetracarboxylic acid dianhydride (BTDA),hexafluoroisopropylidene diphthalic acid dianhydride (6-FDA) and thelike.

The content of the aromatic cyclic acid dianhydride is preferably in therange of 1˜99 mole %, preferably 10˜80 mole % and more preferably 10˜50mole %, based on the total amount of the acid dianhydrides. When thecontent of the aromatic cyclic acid dianhydride is less than the rangedefined above, the final alignment film is poor in mechanical propertiesand heat resistance. On the other hand, when the content of the aromaticcyclic acid dianhydride exceeds the range, the electrical properties,such as voltage holding ratio, of the final alignment film is worsened.

The addition of the alicyclic acid dianhydride which is used to preparethe polyamic acid according to the present invention solves problemssuch as insolubility in common organic solvents, low transmittance inthe visible light region due to the formation of a charge transfercomplex, poor electrooptical properties due to high polarity, resultingfrom the molecular structure of the polyamic acid, etc. Examples of thealicyclic acid dianhydride include, but are not limited to,5-(2,5-dioxotetrahydrofuryl)-3-methylcyclohexene-1,2-dicarboxylic aciddianhydride (DOCDA), bicyclooctene-2,3,5,6-tetracarboxylic aciddianhydride (BODA), 1,2,3,4-cyclobutane tetracarboxylic acid dianhydride(CBDA), 1,2,3,4-cyclopentanetetracarboxylic acid dianhydride (CPDA),1,2,4,5-cyclohexanetetracarboxylic acid dianhydride (CHDA) and the like.

The content of the alicyclic acid dianhydride is in the range of 1˜99mole %, preferably 20˜90 mole % and more preferably 50˜90 mole %, basedon the total amount of the acid dianhydrides.

The polyamic acid of the present invention exhibits good solubility ingeneral aprotic polar solvents such as N-methyl-2-pyrrolidone (NMP),γ-butyrolactone (GBL) dimethylformamide (DMF), dimethylacetamide (DMAc)and tetrahydrofuran (THF). It was found by the present inventors thatsuch increase in solubility is attributed to the introduction of thealicyclic acid dianhydride, and the presence of the main chaincontaining a highly polar triazine moiety and side chains expanding thefree volume of the polymer in the functional diamine. As liquid crystaldisplay devices have recently become large-sized and need to have highresolution and high quality, the printability of aligning agents hasbeen gaining importance. Under these circumstances, good solubility insolvents and high polarity of the polyamic acid are very criticalfactors and have a positive effect upon the printability on a substratewhen the polyamic acid is applied to liquid crystal alignment films.

The polyamic acid of the present invention has a number averagemolecular weight ranging from 10,000 to 500,000 g/mol and preferably10,000 to 300,000 g/mol. The polyamic acid has a glass transitiontemperature of 200˜350° C., which is dependent on the imidization rateor the structure of the polyamic acid.

The polyimide-based liquid crystal alignment film is produced bydissolving the polyamic acid in a given solvent to obtain a liquidcrystal aligning agent, coating the aligning agent onto a substrate, andentirely or partially imidizing the coating.

Examples of solvents used to produce the liquid crystal aligning agentinclude, but are not specially limited to, aprotic polar solvents suchas N-methyl-2-pyrrolidone (NMP), γ-butyrolactone (GBL),dimethylformamide (DMF), dimethylactamide (DMAc), tetrahydrofuran (THF)and the like.

After coating, the imidization reaction is carried out in an oven or ona hot plate at a high temperature of 180˜250° C. for 5˜30 minutes. Theimidization rate may be varied within the range of 30˜99% according tothe intended purpose.

The liquid crystal alignment film produced by imidizing the polyamicacid has a high transmittance of 90% or more in the visible lightregion, excellent alignment properties of a liquid crystal, andfacilitates the pretilt angle control of a liquid crystal within therange of 1˜90°. In addition, since the liquid crystal alignment film ofthe present invention contains the diamine compound of Formula 1, it hasimproved electrooptical properties, e.g., a low refractive index and alow dielectric constant.

Hereinafter, the constitution and effects of present invention will bedescribed in more detail with reference to the following preferredexamples and comparative example. However, these examples are given forthe purpose of illustration and are not to be construed as limiting thescope of the invention.

PREPARATIVE EXAMPLE 1 Preparation of2,4-dichloro-6-hexadecyl-1,3,5-triazine

2.7 g of magnesium was added to 200 ml of tetrahydrofuran and stirreduntil the magnesium was completely dissolved. To the solution was addeddropwise a solution of bromohexadecane (31.1 g) in tetrahydrofuran (200ml). Thereafter, the resulting solution was heated to 65° C. and reactedfor 6 hours with stirring. The reaction solution was slowly addeddropwise to a solution of cyanuric chloride (15 g) in tetrahydrofuran(250 ml), and reacted for 12 hours. After completion of the reaction,the resulting solution was evaporated under reduced pressure to obtain aconcentrate and then the concentrate was purified by distillation invacuo to yield pure 2,4-dichloro-6-hexadecyl-1,3,5-triazine.

PREPARATIVE EXAMPLE 2 Preparation of2,4-dinitrophenoxy-6-hexadecyl-1,3,5-triazine

A solution of the compound (9.7 g) prepared in Preparative Example 1 indichloromethane (100 ml) was added to an aqueous solution of 21.62 g of4-nitrophenol and 6.22 g of sodium hydroxide. The resulting solution wasrefluxed for 24 hours. After the reaction, the aqueous layer wasseparated and the dichloromethane layer was extracted with 1N aqueoussolution, washed with ultrapure water several times, dried overmagnesium sulfate, and evaporated under reduced pressure to obtain acrude product. The crude product was processed by anhydrousrecrystallization to afford2,4-dinitrophenoxy-6-hexadecyl-1,3,5-triazine as a white solid.

PREPARATIVE EXAMPLE 3 Preparation of2,4-diaminophenoxy-6-hexadecyl-1,3,5-triazine

14.5 g of 2,4-dinitrophenoxy-6-hexadecyl-1,3,5-triazine prepared inPreparative Example 2 was dissolved in 300 ml of tetrahydrofuran, andthen 1.4 g of Pd/C was added thereto. Hydrogen gas at 50 psi was fed tothe mixture, and then reacted at 60° C. for 12 hours. After thereaction, the reaction mixture was filtered to remove palladium, andevaporated under reduced pressure to obtain a crude product. The crudeproduct was recrystallized to afford pure2,4-diaminophenoxy-6-hexadecyl-1,3,5-triazine as a white solid. Thefinal product was relatively stable under ambient conditions. Thestructure of the final product was identified through ¹H-NMR spectrum,and the differential scanning calorimetry (DSC) was performed (see,FIGS. 1 and 2).

EXAMPLE 1

0.95 moles of 4,4-methylenedianiline as a diamine component and 0.05moles of 2,4-diaminophenoxy-6-hexadecyl-1,3,5-triazine prepared inPreparative Example 3 were charged into a four-neck flask equipped witha stirrer, a thermostat, a nitrogen injection device and a condenserwhile passing nitrogen, and then N-methyl-2-pyrrolidone (NMP) wasdissolved therein. To the resulting solution was added 0.5 moles of5-(2,5-dioxotetrahydrofuryl)-3-methylcyclohexane-1,2-dicarboxylic aciddianhydride (DOCDA) in a solid form and 0.5 moles of pyromellitic aciddianhydride (PMDA) as acid dianhydride components. The resulting mixturewas vigorously stirred. At this time, the solid content was 15% byweight. The mixture was reacted at a temperature lower than 25° C. for24 hours to prepare a polyamic acid solution (PAA-1) (molecular weight:50,000˜150,000 g/mol).

The polyamic acid solution was applied onto an ITO glass substrate to athickness of 0.1 μm, and cured at 210° C. for 10 minutes to produce aliquid crystal alignment film. After the liquid crystal alignment filmwas subjected to a rubbing process, the alignment properties and thepretilt angle of the liquid crystal were measured. In order to evaluatethe printability of the liquid crystal alignment film, after thealignment film was applied onto an ITO glass substrate, thespreadability and curling properties at the ends were observed.Specifically, the surface of the alignment film was rubbed by means of arubbing machine, two substrates were arranged parallel to each other insuch a manner that the rubbing direction of the each substrate wasreverse, and the two substrates were sealed while maintaining a cell gapof 80 μm to fabricate a liquid crystal cell. The liquid crystal cell wasfilled with a liquid crystalline compound (Merk licristal). Thealignment properties of the liquid crystal were observed under anorthogonally polarlized optical microscope. The pretilt angle of theliquid crystal was measured by a crystal rotation method. The resultsare shown in Table 1 below.

In order to examine the electrical properties, a polyamic acid solutionwas applied onto the surface of an ITO-patterned glass substrate to athickness of 0.1 μm, two substrates were arranged orthogonally to eachother in such a manner that the rubbing direction-of the each substratewas perpendicular (90°, and the two substrates were sealed whilemaintaining a cell gap of 5 μm to fabricate a test cell. The voltageholding ratio was measured, and the results are shown in Table 1 below.

EXAMPLE 2

A polyamic acid (PAA-2) was prepared in the same manner as in Example 1,except that 0.9 moles of 4,4-methylenedianiline and 0.1 moles of2,4-diaminophenoxy-6-hexadecyl-1,3,5-triazine were used. The alignmentproperties, the pretilt angle and the voltage holding ratio weremeasured. The results are shown in Table 1 below.

EXAMPLE 3

A polyamic acid (PAA-3) was prepared in the same manner as in Example 1,except that 0.5 moles of cyclobutane dianhydride was used instead of 0.5moles of 5-(2,5-dioxotetrahydrofuryl)-3-methylcyclohexane-1,2-carboxylicacid dianhydride (DOCDA). The alignment properties, the pretilt angleand the voltage holding ratio were measured. The results are shown inTable 1 below.

EXAMPLE 4

A polyamic acid (PAA-4) was prepared in the same manner as in Example 2,except that 0.5 moles of cyclobutane dianhydride was used instead of 0.5moles of 5-(2,5-dioxotetrahydrofuryl)-3-methylcyclohexane-1,2-carboxylicacid dianhydride (DOCDA). The alignment properties, the pretilt angleand the voltage holding ratio were measured. The results are shown inTable 1 below.

EXAMPLE 5

A polyamic acid (PAA-5) was prepared in the same manner as in Example 1,except that 4,4-methylenedianiline was not used, and 1.0 mole of2,4-diaminophenoxy-6-hexadecyl-1,3,5-triazine was used. The alignmentproperties, the pretilt angle and the voltage holding ratio weremeasured. The results are shown in Table 1 below.

COMPARATIVE EXAMPLE 1

A polyamic acid (PAA-6) was prepared in the same manner as in Example 2,except that 2,4-diamino-1-hexadecyloxybenzene was used instead of2,4-diaminophenoxy-6-hexadecyl-1,3,5-triazine. The alignment properties,the pretilt angle and the voltage holding ratio were measured asdescribed in Example 2. The results are shown in Table 1 below. TABLE 1Voltage holding ratio (%) Pretilt Room Print- Alignment Sample angleTemp. 60° C. ability properties Spreadability PAA-1 5.3 99.5 98.2 GoodGood Good PAA-2 8.1 99.3 97.6 Good Good Good PAA-3 4.8 99.1 97.3 GoodGood Good PAA-4 7.7 99.1 95.9 Good Good Good PAA-5 89.8 99.0 94.5 PoorGood Good PAA-6 6.9 98.8 93.7 Good Good Poor

As can be seen from Table 1, the liquid crystal alignment filmsaccording to the present invention have excellent electroopticalproperties, e.g., high voltage holding ratio, and the pretilt angle ofliquid crystals can be easily controlled.

Although the preferred embodiments of the present invention have beendisclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

1. A diamine compound containing a triazine moiety, represented byFormula 1 below:

wherein A is a direct bond, —O— or —COO—; B is a direct bond, —O—,—COO—, —CONH— or —OCO—; and C is a C_(1˜30) linear, branched or cyclicmonovalent organic group, or a combined form thereof.
 2. The diaminecompound according to claim 1, wherein the substituent C in Formula 1 isa linear or branched aliphatic hydrocarbon group, a saturated cyclichydrocarbon group, a cyclic hydrocarbon group containing at least onecarbon-carbon double bond, or a fused saturated or unsaturated cyclichydrocarbon group which is unsubstituted or substituted with at leastone group selected from the group consisting of —H, —CH₃, —CF₃, —F, —Br,—Cl, —CN, —OH and —NO₂; or a group selected from the following groups:

wherein X₁ and X₂ are each independently —H, —CH₃, —CF₃, —F, —Br, —Cl,—CN, —OH, or —NO₂.
 3. A polyamic acid prepared by reacting a diaminecomponent (a) and an acid dianhydride (b), the diamine componentincluding 0.1 mole % or above of the diamine compound according to claim1 or 2 based on 100 mole % of the diamine component, and the polyamicacid having a repeating unit represented by Formula 2 below:

wherein x is a tetravalent aromatic or alicyclic organic group, and z isa divalent organic group originating from the diamine compound ofFormula 1 or a divalent organic group originating from an aromatic orpolysiloxane-based diamine.
 4. The polyamic acid according to claim 3,wherein the diamine component (a) further includes an aromatic diaminecompound and a polysiloxane-based diamine compound represented byFormulae 3 and 4 below, respectively:H₂N—Y—NH₂   (3) wherein Y is a divalent aromatic organic group,

wherein R₁, R₂, R₃ and R₄ are each independently a C_(1˜10) alkyl,alkoxy or aryl group, and R₅ and R₆ are each independently a C_(1˜10)alkylene group.
 5. The polyamic acid according to claim 4, wherein thesubstituent Y in Formula 3 is a divalent organic group selected from thegroup consisting of the following groups:


6. The polyamic acid according to claim 3, wherein the acid dianhydridecomponent (b) is an aromatic cyclic acid dianhydride represented byFormula 5 below:

wherein X is a tetravalent aromatic cyclic organic group; an alicyclicacid dianhydride represented by Formula 6 below:

wherein X′ is a tetravalent alicyclic organic group; or a mixturethereof, the mixing molar ratio of the aromatic cyclic acid dianhydrideto the alicyclic acid dianhydride being between 1:99 and 99:1.
 7. Thepolyamic acid according to claim 6, wherein the substituent X in Formula5 is a group selected from the following groups:

the substituent X′ in Formula 6 is a group selected from the followinggroups:

wherein X₁, X₂, X₃ and X₄ are each independently —H, —CH₃, —CF₃, —F,—Br, —Cl, —CN, —OH or —NO₂.
 8. The polyamic acid according to claim 3,wherein the polyamic acid has a number average molecular weight rangingfrom 10,000 to 500,000 g/mol.
 9. A liquid crystal aligning agentcomprising the polyamic acid according to claim
 3. 10. A liquid crystalalignment film produced by coating the liquid crystal aligning agentaccording to claim 9 onto a substrate, and entirely or partly imidizingthe coating.
 11. A liquid crystal display device comprising the liquidcrystal alignment film according to claim 10.